The present invention relates generally to intervertebral disk prostheses, and more particularly to improvements in and anchoring systems for disk prostheses to assure operation, fixation and stabilization of the prosthesis corresponding to a natural disk.
The human backbone (the vertebral or spinal column) 10 (FIG. 1) consists of numerous longitudinally aligned vertebrae 11, adjacent ones of which are separated by individual cartilaginous intervertebral disks 12 and connected to one another by ligaments. The vertebral column is the vertical axis of the skeleton, extending from the skull (not show) at its proximal end to the pelvis (not shown) at the distal end. It serves to support the head and trunk of the body, and to protect the spinal cord (not shown) that passes through the vertebral (spinal) canal formed by openings in the vertebrae. Adjacent the distal end of the vertebral column, the sacrum portion 13 of the pelvis is formed by several vertebrae that are fused together and to which the coccyx (tailbone) 14 is attached.
The flexibility of the overall vertebral column allows movements of the trunk in flexion or bending forward, lateral flexion or bending sideways, extension or bending backward, rotation about its longitudinal axis, and circumduction, which is a combination of the aforementioned movements. However, these movements are attributable to the cumulative effect of the numerous small movements that take place at the joints between the vertebrae.
As shown in FIG. 1, the vertebral column 10 has an upper cervical curvature 15, a middle thoracic curvature 16, a lower lumbar curvature 17, and a lowest pelvic curvature 18. The cervical region comprises seven vertebrae, the thoracic region twelve vertebrae, and the lumbar region five vertebrae (for convenience, all of the vertebrae are depicted by reference number 11), with various common characteristics as well some different features according to the functions the respective vertebrae serve. For example, the first cervical vertebrae is configured to support and balance the head, and the second is configured to pivot within the first as the head is turned from side to side, respectively. These vertebrae are atypical, possessing certain structural features not found in the others. The thoracic vertebrae have larger bodies than the cervical vertebrae to accommodate increased stress as a result of their support of successively increasing body weight, and the lumbar vertebrae still larger bodies for the same reason.
A typical vertebra 11 is depicted from above in FIG. 2, which illustrates the bony vertebral body 20 anteriorly of the intervertebral column 10, aligned along the respective curvatures with the bodies of the other vertebrae of the column. A bony vertebral arch is formed by a pair of posteriorly projecting pedicles 21, 22, and laminae 23, 24 extending from the pedicles and joined together as the spinous process 25, the arch surrounding a vertebral foramen 26 aligned with corresponding foramen of the other vertebrae to form a vertebral or spinal canal for passage of the spinal cord (not shown). Transverse processes 28, 29 project from between the pedicle 21 and the lamina 23 on one side, and 22 and 23 on the other, respectively. Superior 30, 31 and inferior (hidden from view in FIG. 2) articular processes have cartilage covered facets for joining adjacent vertebrae above and below to the vertebra shown in FIG. 2. Notches on the lower portion of the pedicles 21, 22 create intervertebral foramina openings (34, FIG. 1) through which spinal nerves (not shown) pass between adjacent vertebrae to connect to the spinal cord.
The facet joints on each side between the articular processes of adjacent vertebrae constitute two of the three separate intervertebral joints between each vertebra and the adjacent vertebra above (or below) it. The third is the anterior joint formed between the bodies of adjacent vertebrae by the intervertebral disk 12 which both unites them and allows movement between them. The type and degree of movement of the vertebral column acting as a unit is controlled by the actions of all three intervertebral joints between the separate vertebrae, allowing, for example, pure rotation of the column only in the thoracic region.
The intervertebral disks 12 separating the vertebrae are masses of fibrocartilage that cushion and soften forces arising from movements such as walking and jumping, as well as providing one of the joints between the adjacent vertebrae. The disks are shaped according to their locations in the spine. Those in the thoracic region are relatively thinner and flatter than the disks in each of the cervical and lumbar regions, which are wedge-shaped and relatively thicker. Anterior and posterior longitudinal bands of ligamentous fibers extending along the length of the vertebral column and attached to the bodies of the vertebrae serve in part to reinforce the disks in front and behind.
Each disk 12 itself is composed of a tough outer layer of the fibrocartilage and an elastic central region. Injury, exertion or the aging process can produce changes in the disks, such as loss of firmness of the central region and thinning, weakening and cracking of the outer layer as a result of degenerative changes, and breakage of the outer layer and squeezing out of the central region as a result of injury from external pressure or heavy lifting. Pressure on the spinal cord or individual nerves branching from the cord caused by the ruptured or slipped disk often produces back pain, numbness and loss of muscular function in the body parts innervated by the affected spinal nerve(s).
More specifically, each disk has an annulus composed of concentric rings of strong fibers (the annulus fibrosus, or annulus) that surrounds a central gelatinous nucleus (the nucleus pulposus, or nucleus). The annulus fibers are attached in an oblique direction at top and bottom of the disk to the adjacent vertebrae so that some of the fibers tighten when the related vertebrae are rotated in one direction and the others tighten when the rotation is in the opposite direction, to resist torsional motion between vertebral segments and excessive movement in almost any direction. Also, the fibers in adjacent rings are oriented at right angles to enable strong bonding while allowing some movement between the bones. The nucleus is not centered in the disk but resides more toward the posterior, with the annulus thinner in that sector and thicker at the anterior sector. The nucleus is designed for deformation in response to exertion of pressure on the disk so that the disk can change shape during movement of the vertebral column. This accommodates bending of the vertebral column and resulting displacement of confronting surfaces of the adjacent vertebrae from a substantially parallel orientation. In concert, the annulus undergoes stretching in the sector of wider displacement and bulging in the sector of narrower displacement of those opposing surfaces.
Vertebral end plates at opposite ends of the disk abut against the bodies of the respective adjacent vertebrae above and below, operating as a transition zone between the bony vertebrae and the soft intervertebral disk. The disk itself is without blood vessels, so it receives its nutrients for metabolism by diffusion through the end plates.
The intervertebral disk can become herniated when the fibers of the annulus weaken or tear as a result of abnormal or repeated stress or because of degenerative processes with aging. The nucleus then tends to become distended and unable to recover to its normal position within the annulus. In such cases, nerve compression can occur as the bulging disk penetrates the vertebral canal and begins exerting ongoing pressure on the spinal cord or on individual nerves that pass between adjacent vertebrae and connect to the spinal cord. This occurs most often with disks located in the lumbar region, where the greatest stresses attributable to weight are present. The result is chronic lower back or leg pain, which can be disabling.
A common procedure in such cases, typically after first having attempted a conservative approach with treatment regimens of anti-inflammatory drugs, patient rest, or physical therapy or a combination thereof, without significant success, is to surgically remove the defective disk, implant patient or donor bone, and/or fuse the adjacent vertebrae, so as to alleviate the pain at least to an extent.
While fusion enjoys success in alleviating symptoms and stabilizing the joint at the previous vertebra-disk interface, it decreases the range of motion of the vertebral column in the portion of the region where the surgical procedure was performed. Also, the biomechanical rigidity of the fused vertebrae may exacerbate deterioration of adjacent portions of the region.
Artificial intervertebral disks or disk prostheses offer replacement of the defective natural intervertebral disk with a capability of performing many of the functions of the latter, at least to an extent to reduce problems suffered as a result of the defects, and an opportunity to avoid further degeneration of the vertebral column.
Artificial intervertebral disks, partial disk prostheses and techniques of fixation thereof have been proposed in several United States patents, including the following.
U.S. Pat. No. 3,867,728 discloses a synthetic kidney shaped prosthetic disk with a core (nucleus) composed of biocompatible viscoelastic liquid or elastomer contained in a sealed chamber such that the core resists deformation under compressive loading. The walls of the chamber are surrounded by medical elastomer layers reinforced with embedded fibrous material such as Dacron filaments. The fibrous material is intended to act as an open-pore, tissue ingrowth-receptive surface that abuts the exposed bony surface of an adjacent vertebra when resident between the natural surfaces of the vertebral cavity from which the replaced disc was excised. The elastomeric core is reinforced with an annular ring of laminated fibrous material embedded in the elastomer. The biocompatible viscoelastic liquid core has reinforced side and end walls of medical grade elastomer with embedded fibrous material to provide a sealed chamber for the liquid, and an open pore tissue-ingrowth receptive surface positioned to abut the exposed bony surface of an adjacent vertebra.
U.S. Pat. No. 4,772,287 discloses a prosthetic disc capsule for repairing a natural herniated disk. The prosthesis has an outer layer composed of strong inert fibers that surrounds a bladder containing a thixotropic gel having a viscosity and velocity shear behavior imitating that of a natural spinal disc. One or more of the capsules are inserted into bores formed in the natural herniated disc under repair. The inert fibers of the outer layer are composed of carbon or a polymer, including either natural or synthetic polymers such as cold-drawn poly(ethylene terephthalate) polyester fibers, or of bioresorbable material consisting of polylactic or polyglycolic acid or collagen (e.g., semi-synthetic), for replacement by tissue ingrowth for bonding to surrounding tissue. The bladder is flexible, composed of oriented poly(ethylene terephthalate), high-density polypropylene, silicone rubber, and copolymers of silicone and carbonate. The thixotropic gel is a mixture of an inorganic oil such as silicone or fluorocarbon, and a gelling agent such as fumed silica. The viscosity of the gel is selected to permit fast movement during bending at the intervertebral space while restricting motion during slow postural changes.
U.S. Pat. No. 4,946,378 discloses an artificial intervertebral disc with a pair of metallic end bodies having 0.1 to 0.5 mm hydroxyapatite layer-coated outer surfaces and a biocompatible synthetic elastic polymeric intermediate material of silicone rubber, polyvinyl alcohol, polyurethyane resin, or the like held between the end bodies through connecting members composed of titanium, stainless steel, or the like.
U.S. Pat. No. 5,047,055 discloses a prosthetic lumbar disk nucleus fabricated from synthetic hydrogels selected because of their biocompatibility, characteristics of softness, hydration, low friction, viscoelasticity, shape memory, and mechanical strength which can aid the healing of a defective annulus of the intervertebral disk. The prosthetic nucleus may purportedly be implanted in the dehydrated state laterally to reduce the complexity and risk of traditional intraspinal surgery, and allowed to swell slowly in the body thereafter, with reduced incision area on the annulus to aid healing of the annulus and prevent herniation of the disk. The implanted hydrogel nucleus, after hydration, is constrained tightly in the cavity formed by the excised natural nucleus, by the restoring force of stretched fibers of the annulus and the external force through the end plates.
U.S. Pat. No. 5,192,326, a continuation-in-part of the '055 patent, also discloses a prosthetic lumbar disk nucleus, but in which the nucleus is composed of hydrogel beads.
U.S. Pat. No. 5,258,043 discloses a prosthetic disk is composed of a dry, porous, volume matrix of biocompatable and bioresorbable fibers, a portion of which may be cross-linked, structured for implantation to assume the form and role of a natural intervertebral disk. The matrix may promote regrowth of intervertebral fibrochondrocytes and provides a scaffold for the regenerating intervertebral disk tissue. The prosthetic disc may further include a mesh composed of a bioresorbable, biocompatible material attached to lateral portions of the outer surface of the matrix to aid disk implantation by providing a temporary anchoring mechanism.
U.S. Pat. No. 5,458,643 discloses a prosthesis in the form of an artificial intervertebral disk composed of polyvinyl alcohol (PVA) hydrogel and a porous ceramic or metal. The PVA hydrogel is said to enhance lubrication and shock absorbing functions, and the porous body to allow the ingrowth and ossification of adjacent bone tissue of the body in which the prosthesis is implanted.
U.S. Pat. No. 5,824,093, related to the '287 patent cited above, discloses a prosthetic disk capsule to be implanted in pairs side-by-side in a damaged natural intervertebral disk to maintain both height and motion. Each capsule is an elongated, prosthetic spinal disc nucleus body composed of a hydrogel core and a surrounding constraining jacket that permits the hydrogel core to deform and reform and to hydrate to a predetermined volume with deformation and reformation in response to various loads placed upon the spinal tract.
At least some of these artificial intervertebral disks present the possibility of successful implants on a practical scale. However, so far as is known to the applicant herein, the prior art has not given promise of successful implementations and methods for anchoring implanted artificial intervertebral disks in the vertebral column to achieve relatively permanent fixation and stabilization of the implant.
U.S. Pat. No. 5,562,738 discloses an intervertebral disk arthroplasty device for replacing a degenerated or ruptured intervertebral disk. The disk includes a first member with a socket portion and a second member with a ball portion fitting in the socket portion. The first member fits adjacent the first vertebrae and the second member fits adjacent the second vertebrae so that the ball portion fits in the socket portion, in the space vacated by the excised disk. The members are anchored in place by base plates with tabs fastened to the members, and by screws through the tabs into the adjacent vertebrae. A second embodiment utilizes metal insert cups fastened to the members, and another embodiment contemplates bone ingrowth into ceramic members.
Prior art artificial intervertebral disks, and techniques of fastening, anchoring and stabilizing implanted artificial disks do not appear likely to yield satisfactory results.
It is a primary aim of the present invention to provide improved intervertebral disk prostheses, and anchoring systems that achieve reliable stabilization and relatively permanent fixation of the improved intervertebral disk prosthesis in a vertebral column.
Another aim of the invention is to provide methods of fabricating intervertebral disk prostheses, and of anchoring the prosthesis, for achieving those results.